The present invention relates to a sealing cover for a hermetically sealed container and, more particularly, relates to a method of attaching a preformed solder ring to a cover used to hermetically seal the container of a semiconductor device.
In order to operate effectively, a semiconductor device is hermetically sealed in a container or can. Numerous means have been devised to provide this enclosure or encapsulation. One of the simplest is to assemble the device within an open top container with side walls having coplanar edges to which a flat cover having corresponding peripheral dimensions can be soldered. Solder must be provided in a form and quantity which may be melted without danger of excess solder flowing into the container and damaging the device. This may be done by stamping from sheet of desired thickness a solder ring having outer dimensions which correspond to that of the flat cover and having a width sufficient to ensure contact with the planar upper edge of the container. The preformed solder ring is usually rectangular or circular in shape. Placement of such a solder ring between the container edge and the cover followed by heating to melt the solder and cooling to solidify the solder to bond the lid to the cover is well known. However, this method requires handling and alignment of three components and rapid assembly is difficult to achieve.
Various means have been devised whereby the final assembly of a container for a semiconductor device can be reduced to handling of only two components.
In the method of U.S. Pat. No. 3,538,597, a thin coating of tin-lead eutectic solder is applied by plating or cladding to a KOVAR (Trademark of Westinghouse Electric Corp. for a cobalt-nickel-iron alloy which may contain about 17% cobalt and about 20% nickel) sheet from which covers are punched. The cover is placed on the housing and the assembly is heated to melt the solder. The solder flows into the area of contact and solidifies to bond the cover to the housing.
In the method of U.S. Pat. No. 3,579,817, a fusible alloy border is deposited onto a circuit package lid in the molten state or by such known methods as brushing, spraying or screening. A dielectric material fills the inner portion of the lid not covered by the fusible alloy border. If thicker than the fusible alloy border, the dielectric material may serve as a means to align the cover with the open frame of the circuit package.
U.S. Pat. No. 3,648,357 provides a method of hermetically sealing flat pack containers having KOVAR cans and covers, both of which are gold plated on their sealing surfaces. Relatively thick gold layers, 250 to 400 microinches or 0.25 to 0.4 mils, are applied. In addition, these coatings are each partially dissolved into an eutectic gold-tin solder preform at a temperature above the eutectic point to provide pre-tinning of the surfaces. The pre-tinned surfaces are then placed together and fused to seal the container.
U.S. Pat. No. 3,946,190, a division of U.S. Pat. No. 3,874,549 which is a division of U.S. Pat. No. 3,823,468, provides a method of fabricating a hermetic sealing cover in which a preformed solder ring is superimposed on the cover and fused to the cover at spaced points, fusion by spot welding being disclosed. Registry of the preformed solder ring with the periphery of the cover may be effected by enclosure within a shallow cavity of a nonconductive supported member. U.S. Pat. No. 3,874,549 claims a hermetic sealing cover wherein the solder ring is fused to the cover element at a plurality of spaced points.
U.S. Pat. No. 3,937,388 provides a method of sealing a pair of elements together to form a package which comprises applying a meltable sealant to a portion of one of the elements, placing one of the elements on a first heat sink and the other element on top of the first element. Heat is applied to both elements to melt the sealant and cause it to flow between the elements in a sealing relation. A second heat sink is applied to the second element to remove heat from the elements during application of heat to the sealant. Pressure is applied and heating is discontinued. Heat is removed by both heat sinks both during and after discontinuation of the heating.